During the drilling and completion of oil and gas wells, it may be necessary to engage in ancillary operations, such as evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested or sampled to determine various formation properties such as permeability, fluid type, fluid quality, formation temperature, formation pressure, bubblepoint and formation pressure gradient. These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable and how to optimize production. The acquisition of accurate data from the wellbore is critical to the optimization of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations.
Frequently, adsorbing chemicals or corrosive substances occur in the hydrocarbon reservoirs. Adsorption is the adhesion of molecules (as gases, solutes, or liquids) in a thin layer to the surface of a solid or liquid. Thus, an adsorbing chemical is a gas, liquid, solute, or suspension that is attracted to and held by the surface of a solid or other liquid. Desorption is to removed by reverse adsorption, or the release of the molecules from the solid or liquid surface.
For example, Hydrogen Sulfide (H2S) may be present as the result of reactions involving sulfur containing materials such as Kerogen or Anhydride (CaSO4), or as the result of the reaction between anaerobic bacteria and organic matter in the reservoir. The resulting levels of H2S can vary, with an exemplary range being 5 to 40 percent concentration. H2S is problematic for several reasons. It is a health hazard. H2S is also corrosive to drilling and production equipment. H2S can react with downhole equipment and the formation to produce scale, which then impedes or closes off the production flow line or the permeability of the formation. Steps taken to address these known problems with H2S and other corrosive substances increase the overall cost of drilling and production. Further, H2S containing hydrocarbons (sour oil) is less valuable than hydrocarbons without H2S (sweet oil). While H2S is an exemplary corrosive, adsorbing chemical, other adsorbing chemicals can similarly affect downhole operations and production.
Different levels of H2S in the petroleum reservoir dictate different levels of H2S mitigation and different valuations of the oil. To properly project the economics and well plan, the level of H2S in the reservoir must be accurately known.
To obtain downhole concentration levels of H2S, the formation is tested or sampled for the level of H2S therein. A formation tester or other measurement device is placed in the borehole to test or capture a sample of fluid from the formation. In some cases, the formation tester directly measures H2S optically, electrically, or chemically. In other cases, the formation tester receives and captures a fluid sample for analysis in the tool or subsequently at the surface of the well. The formation tester may be conveyed downhole a number of ways. The tool may be used in conjunction with wireline logging operations or as a component of a logging-while-drilling (LWD) or measurement-while-drilling (MWD) package. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable (wireline) that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board or remote control systems. With LWD/MWD testers, the testing equipment is subject to harsh conditions in the wellbore during the drilling process that can damage and degrade the formation testing equipment before and during the testing process. These harsh conditions include vibration and torque from the drill bit, exposure to drilling mud, drilled cuttings, and formation fluids, hydraulic forces of the circulating drilling mud, high downhole temperatures, and scraping of the formation testing equipment against the sides of the wellbore. Sensitive electronics and sensors must be robust enough to withstand the pressures and temperatures, and especially the extreme vibration and shock conditions of the drilling environment, yet maintain accuracy, repeatability, and reliability.
Regardless of the testing tool configuration, interaction of adsorbing chemicals with the tool presents inaccuracies in the adsorbing chemical measurement and analysis, as described more fully herein. The principles of the present disclosure overcome the limitations of the prior art in accurately measuring adsorbing or corrosive chemicals such as H2S.